1. Field of the Invention
The present invention relates generally to a projection apparatus implemented with a micromirror device. More particularly, this invention relates to a projection apparatus implemented with a micromirror device combining with a control process for adjusting an aperture ratio of a projection optical system to further increase the gradations of gray scales for improving quality of the image display.
2. Description of the Related Art
Even though there have been significant advances made in recent years on the technologies of implementing electromechanical micromirror devices as spatial light modulator, there are still limitations and difficulties when employed to provide high quality images display. Specifically, when the display images are digitally controlled, the image qualities are adversely affected due to the fact that the image is not displayed with a sufficient number of gray scales.
Electromechanical micromirror devices have been drawing considerable interest because of their application as spatial light modulators (SLMs). A spatial light modulator requires an array of a relatively large number of micromirror devices. In general, the number of devices required ranges from 60,000 to several million for each SLM. Referring to FIG. 1A, a digital video system 1 including a display screen 2 as disclosed in a relevant U.S. Pat. No. 5,214,420. A light source 10 is used to generate light energy for ultimate illumination of display screen 2. Light 9 generated is further concentrated and directed toward lens 12 by mirror 11. Lens 12, 13 and 14 form a beam columnator to operative to columnate light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer 19 through data transmitted over data cable 18 to selectively redirect a portion of the light from path 7 toward lens 5 to display on screen 2. The SLM 15 has a surface 16 that includes an array of switchable reflective elements, e.g., micromirror devices 32, such as elements 17, 27, 37, and 47 as reflective elements attached to a hinge 30 that shown in FIG. 1B. When element 17 is in one position, a portion of the light from path 7 is redirected along path 6 to lens 5 where it is enlarged or spread along path 4 to impinge the display screen 2 so as to form an illuminated pixel 3. When element 17 is in another position, light is not redirected toward display screen 2 and hence pixel 3 would be dark.
The on-and-off states of micromirror control scheme as that implemented in the U.S. Pat. No. 5,214,420 and by most of the conventional display system imposes a limitation on the quality of the display. Specifically, when applying conventional configuration of control circuit has a limitation that the gray scale of conventional system (PWM between ON and OFF states) is limited by the LSB (least significant bit, or the least pulse width). Due to the On-Off states implemented in the conventional systems, there is no way to provide shorter pulse width than the LSB. The smallest brightness, which determines the gray scale, is the light reflected during the smallest pulse width. The limited gray scale leads to degradations of image display.
Specifically, in FIG. 1C an exemplary circuit diagram of a prior art control circuit for a micromirror according to U.S. Pat. No. 5,285,407. The control circuit includes memory cell 32. Various transistors are referred to as “M*”, where * designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5, and M7 are p-channel transistors; transistors, M6, M8, and M9 are n-channel transistors. The capacitances, C1 and C2, represent the capacitive loads presented to memory cell 32. Memory cell 32 includes an access switch transistor M9 and a latch 32a, which is the basis of the static random access switch memory (SRAM) design. All access transistors M9 in a row receive a DATA signal from a different bit-line 31a. The particular memory cell 32 to be written is accessed by turning on the appropriate row select transistor M9, using the ROW signal functioning as a wordline. Latch 32a is formed from two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states, that is, state 1 is Node A high and Node B low and state 2 is Node A low and Node B high.
The dual states switching as illustrated by the control circuit controls the micromirrors to position either at an ON or OFF angular orientation as that shown in FIG. 1A. The brightness, i.e., the gray scales of display for a digitally control image system, is determined by the length of time the micromirror stays at an ON position. The length of time a micromirror controlled at an ON position is in turn controlled by a multiple bit word. For simplicity of illustration, FIG. 1D shows the “binary time intervals” when control by a four-bit word. As that shown in FIG. 1D, the time durations have relative values of 1, 2, 4, 8 that in turn define the relative brightness for each of the four bits, where 1 is for the least significant bit and 8 is for the most significant bit. According to the shown control mechanism, the minimum controllable difference between gray scales for showing different brightness is brightness represented by a “least significant bit” that maintains the micromirror at an ON position.
When adjacent image pixels are shown with a great variance in gray scales due to the controllability of gray scales being very coarse, artifacts are shown between these adjacent image pixels. This leads to image degradations. The image degradations are especially pronounced in bright areas of display when there are “bigger gaps” of gray scales between adjacent image pixels. It was observed in an image of a female model that there were artifacts shown on the forehead, the sides of the nose and the upper arm. The artifacts are generated due to a technical limitation when the digitally controlled display does not provide sufficient gray scales. At the bright spots of display, e.g., the forehead, the sides of the nose and the upper arm, the adjacent pixels are displayed with visible gaps of light intensities.
As the micromirrors are controlled to have a fully on and fully off position, the light intensity is determined by the length of time the micromirror is at the fully on position. In order to increase the number of gray scales of display, the speed of the micromirror must be increased such that the digital control signals can be increased to a higher number of bits. However, when the speed of the micromirrors is increased, a strong hinge is necessary for the micromirror to sustain a required number of operational cycles for a designated lifetime of operation. In order to drive the micromirrors supported on a further strengthened hinge, a higher voltage is required. The higher voltage may exceed twenty volts and may even be as high as thirty volts. A micromirror manufacturing process applying the CMOS technologies probably produce micromirrors that would not be suitable for operation at such a higher range of voltages and therefore the DMOS micromirror devices could be required in this situation. In order to achieve a higher degree of gray scale control, a more complicated manufacturing process and larger device areas are necessary when a DMOS micromirror is implemented. Conventional modes of micromirror control are therefore facing a technical challenge due to the fact that the gray scale accuracy has to be sacrificed for the benefit of a smaller and more cost effective micromirror display due to the operational voltage limitations.
There are many patents related to light intensity control. These patents include U.S. Pat. Nos. 5,589,852, 6,232,963, 6,592,227, 6,648,476, and 6,819,064. There are further patents and patent applications related to different shapes of light sources. These patents include U.S. Pat. Nos. 5,442,414, 6,036,318 and Application 2003/0147052. U.S. Pat. No. 6,746,123 discloses special polarized light sources for preventing light loss. However, these patents and patent applications do not provide an effective solution to overcome the limitations caused by insufficient gray scales in the digitally controlled image display systems.
Furthermore, there are many patents related to spatial light modulation, including U.S. Pat. Nos. 2,025,143, 2,682,010, 2,681,423, 4,087,810, 4,292,732, 4,405,209, 4,454,541, 4,592,628, 4,767,192, 4,842,396, 4,907,862, 5,214,420, 5,287,096, 5,506,597, and 5,489,952. However, these inventions have not addressed and provided direct resolutions for a person of ordinary skill in the art to overcome the above-discussed limitations and difficulties.
Therefore, a need still exists in the art of image display systems applying digital control of a micromirror array as a spatial light modulator to provide new and improved systems such that the above-discussed difficulties can be resolved.